1. Field of the Invention
The need to control and optimize the properties of interfaces between solids and thermoset polymeric resins is critical in a host of technologies. Often the adhesive strength of the interface, the resistance to moisture and corrosion (metals), and the ability to transfer stress through the interface are critical to the overall performance of the product. Some examples include layers of conducting metals and polymer dielectrics in the electronics packaging industry, adhesive joints between aluminum or stainless steel and epoxy in the aerospace and automotive industries, and carbon black- or silica-filled rubber in the tire industry.
Etching and controlled pre-oxidation treatments are commonly used to promote adhesion and durability of metal/thermoset interfaces. While excellent performance can often be achieved, these processes are time consuming, costly, and involve harsh chemicals detrimental to the environment. In addition, for certain technologies the roughness of the solid surface, which is an essential element of the etching and pre-oxidation treatments, is detrimental and limiting to the product. One such example is the electronic circuit board industry, where copper line spacings and linewidths can become limited by the roughness of commercial foil treatments. Moreover, in the subtractive process of manufacturing circuit boards, copper must be etched away to pattern the circuit. The difficulty of this processing step increases dramatically with the roughness of the copper.
Several obstacles must be overcome to provide a strong, durable bond between a solid surface and a thermoset resin which does not rely upon physical roughness. First, the thermoset resin and the solid surface must be linked through a chain of chemical bonds. This can be especially difficult for certain metals, such as nickel, which are unreactive toward most functional groups. Second, for metals, the surface of the metal must be protected from oxidation and corrosion which result from the combined effects of heat, oxygen, and water. This criterion is especially severe for certain metals, like copper, which oxidize rapidly when heated in air to produce a weak surface-oxide layer hundreds of nanometers thick. This thick, brittle oxide layer leads to failure at low loadings. Third, the properties of the interface region must be such that the stresses that build up during thermal cycling due to differences in coefficient of thermal expansion can be dissipated nondestructively.
Primer formulations involving small-molecule silane coupling agents work extremely well for glass, silica, and certain metal substrates. However, there are many metals and other inorganic substrates to which silane coupling agents will not bond. Another problem with silanes is that they do not protect metal surfaces from oxidation and corrosion. Small-molecule chelating agents have been proposed as a resolution to this latter problem (U.S. Pat. Nos. 3,837,964; 4,428,987; and 4,448,847). These materials, which possess a chelating group which bonds to the metal and another functional group which bonds to the polymer resin, have demonstrated a significant level of corrosion inhibition under ambient conditions. However, in contrast to silane coupling agents, these do not possess the ability to form a three-dimensional interpenetrating network and entangle with the resin. Moreover, they are susceptible to attack by moisture and are not stable at elevated temperature. Polymers containing chelating agents have demonstrated improved passivation at elevated temperature, but with only a small increase in adhesive strength (Eng and Ishida, Hanson, et al.), likely due to limited reactivity of the chelating agent with the thermoset resin.
Recently, polymeric coupling agents have been proposed (U.S. Pat. No. 4,812,363) which possess a single chemical functionality capable of bonding to both the metal and to the thermoset resin. This reactive group is incorporated at set intervals along the backbone of the polymer. The advantages cited in this method are that the backbone of the polymer can be made hydrophobic to provide increased resistance to water, that attachment of the functional groups to a polymefic backbone adds stability and strength to the interphase, and that polymefic coupling agents can be more efficient than low molecular-weight compounds in dissipating stresses which develop due to mismatch in the coefficient of thermal expansion. However, it has not been demonstrated that the level of passivation is sufficient to protect metals like copper from destructive oxidation after exposure to elevated temperatures such as those achieved in soldering operations. In addition, the method involves the use of multilayers which can lead to failure between the layers. Also, the fact that only one type of functional group is present in the polymer makes it unlikely that both functions, bonding to and passivating metal oxide surfaces and bonding to thermosets, can each be accomplished in an optimal fashion for a variety of metals and thermosets. None of the above performs well enough to replace the etching and oxidation treatments for most applications.
2. Background Art
1. R. H. Grubbs and W. Tumas, Science, 243,902 (1989).
2. R.R. Schrock, Ace. Chem Res., 23, 158 (1990).
3. F. P. Eng and H. Ishida, J. Electrothem. Soc. 35, 603(1988).
4. J. Hanson, M. Kamagai, and H. Ishida, Polymer, 35., 4780(1994).
5. U.S. Patent No. 3,837,964.
6. U.S. Patent No. 4,812,363.
7. U.S. Patent No. 4,428,987.
8. U.S. Patent No. 4,448,847.